1. Field of the Invention
The present invention generally relates to multiphase-motor driving and control techniques. More specifically, the invention relates to a motor driving apparatus and a motor driving method for a rotor-position sensorless motor having no rotor position sensor such as a Hall-effect device for sensing a rotor position.
2. Description of Related Art
In recent years, in sensorless driving of a small three-phase motor, switching timing of an energized-phase is controlled by providing a non-energized period (also referred to as “current-OFF period”, hereinafter) in which a coil current of one phase in a “Y” connection (i.e., “star connection”) coil is set to zero. More specifically, energized-phase switching timing is controlled by detecting zero crossings of back electromotive voltages associated with rotor rotation which are generated in an inter-terminal potential difference between both terminals, namely between an energization terminal and a neutral point terminal, of the coil of the phase during the non-energized period.
Conventionally, when abrupt current variations occur in switching of an energized-phase, there arise vibrations and noises to be drawbacks. In order to reduce such vibrations, noises, and the like, for example, Patent document 1, i.e., Japanese Patent No. 2892164 discloses a method for smoothing current variations. FIG. 11 shows a basic circuit configuration as disclosed in the patent document. Referring to FIG. 11, reference numeral 16 denotes a rotor position detector section which includes three comparators 24 corresponding to three phases (U, V, and W phases) and a phase-processing logic circuit 23. The inter-terminal potential differences of the respective motor coils during the non-energized periods are compared by the comparators 24, and converted by the phase-processing logic circuit 23 to a rotor phase information signal.
In the configuration of FIG. 11, three phase driving current waveforms 101, 102, and 103 of a sensorless motor are obtainable as shown in FIG. 12 in a phase-switching trapezoidal wave synthesizer section 21. These three phase driving current waveforms are formed to be smooth as trapezoidal wave shapes, and have non-energized periods Ta, Tb, Tc, Td, Te, and Tf for reading back electromotive voltages of coil terminals in order to detect the rotor position.
In addition, Patent document 2, i.e., Japanese Unexamined Laid-open Patent Publication No. 2003-174789 discloses a technique as outlined herebelow. This configuration includes a PWM (pulsewidth modulation) control section that generates PWM control pulses independent of each other and that performs PWM control, in parallel for two phases, of energization of a phase determined by an energization switching section. The configuration further includes a comparator section that performs comparison between a current detection signal indicative of a level of current flowing to a motor coil and various torque command signals generated by a torque command signal generator section. By determining an ON period of the PWM control pulse, the switching of the phase current is performed smoothly in the range from a low torque to a high torque, thereby reducing the motor vibrations and noises generated due to sharp changes in phase current. More specifically, according to Patent document 2, one phase coil terminal, except for a neutral point 4, is fixed to either one of a high potential and a low potential, and driving transistors of the remaining two phase coil terminals are alternately time-divided to be ON state so as to reach respective target current values or the sum current value thereof. Thus, these two phase coil current values are controlled, and an opposite-sign current obtained by summing the two phase currents is set as the current of the potential-fixed coil.
However, according to these conventional techniques, as shown in FIG. 11 for example, in the “Y”-connected three phase motor coils, there is not provided a driving transistor directly connected to the neutral point 4. In addition, the publication document does not discloses a technique for reducing the vibration and noise of the motor under controlling the coil current waveform of the other two phases in the energized state in the intervals Ta, Tb, Tc, Td, Te, and Tf wherein the coil of only any one of phases is in the non-energized state.
As in the conventional examples shown in FIGS. 11 and 12, even when coil current profiles of the individual phases are simply controlled to be trapezoidal wave shapes provided with non-energized intervals, significant vibrations and the noises are still generated. A reason therefor is that the motor vibration and noise are significantly dependent on components of a force that acts in the motor shaft direction between the rotor and the stator, wherein the current waveform includes a large amount of the vibration components acting in the shaft direction. When the motor rotor is virtually displaced in the shaft direction with respect to the motor stator, the magnetic flux across the respective phase coils is varied. Generally, the variation rate of such magnetic flux has the same waveform as the total magnetic flux across a corresponding phase coil. Hereafter, the magnetic flux variation rate will be referred to as either “magnetic flux variation rate in the motor shaft direction” or “shaft direction force constant”. Different from a force (torque) acting in the rotation direction, the magnetic flux variation rate in the motor shaft direction applies as a force acting in the motor shaft direction, the force acting in the shaft direction is remarkably influenced by the current variation in a time region where the current exhibits a zero crossing. As such, the existence of the non-energized period of a coil causes a residue of shaft-direction vibration components having an amplitude of a non-negligible level, resulting in that sufficient suppression of the vibration, the noise, and the like can not be achieved.
Referring to FIGS. 13A to 13C, factors for not sufficiently suppressing the vibration and the noise of the motor having the non-energized periods in the coil current will be described referring to the case of a three-phase motor as an example. FIG. 13A includes the three phase driving current waveforms 101, 102, and 103, which are the same as those shown in FIG. 12. The three phase driving current waveforms represent first phase (U phase), second phase (V phase), and third phase (W phase) coil current waveforms each having a trapezoidal-wave shaped current waveform. Each of the three phase driving current waveforms 101, 102, 103 has a period wherein the coil current becomes zero, i.e., non-energized state in a period in the vicinity of the zero crossing of the respective current. Ta denotes a non-energized period in a current increasing region included in the coil current of the first phase, Tb denotes a non-energized period in a current increasing region included in the coil current of the second phase, and Tc denotes a non-energized period in a current increasing region included in the coil current of the third phase. Td denotes a non-energized period in a current decreasing region included in the coil current of the third phase, Te denotes a non-energized period in a current decreasing region included in the coil current of the first phase, and Tf denotes a non-energized period in a current decreasing region included in the coil current of the second phase.
It can easily be known that the summation of the respective phase coil currents 101, 102, and 103 results in a current value of zero as shown in FIG. 13A. This is an inevitable consequence of the case without driving means for directly driving the neutral point. Reference numeral 104 represents the waveform of the magnetic flux variation rate (shaft direction force constant) of the first phase with respect to the motor-shaft direction variation rate. The magnetic flux variation rate waveform 104 is approximately represented as a waveform proportional to a sine wave that is different in phase by an electrical angle of 90° from a sine wave component of a fundamental wave of the first phase coil current waveform 101. Generally, the magnetic flux variation rate relative to a displacement in a motor shaft direction can be said to be proportional to a sine waveform that is different in phase by an electrical angle of 90° from the magnetic flux variation rate relative to a displacement in the motor rotation direction. The magnetic flux variation rate relative to the motor-rotation-directional displacement is alternatively called a torque constant, and is distinguished from the shaft direction constant or the magnetic flux variation rate relative to the above-described motor-shaft direction displacement.
As such, a torque constant waveform in units of the respective phase coil current is represented in the form of a sine wave matching in phase with the fundamental wave of the respective phase coil current, and the shaft direction force constant in units of the respective phase coil current is represented in the form of a sine wave 90° delayed in phase from the respective torque constant waveform. The product of the multiplication of the first phase coil current 101 times the magnetic flux variation rate (shaft direction force constant) 104 relative to the motor-shaft direction displacement represents the motor-shaft direction force with respect to the first phase coil current. Although not shown in the drawings, similarly as in the case of the first phase, the second phase magnetic flux variation rate relative to a motor-shaft direction displacement is approximately represented proportional to a sine waveform that is different in phase by an electrical angle of 90° from the second phase coil current 102, and the product of the multiplication of the two values represents a motor-shaft direction force with respect to the second phase.
Similarly, third phase magnetic flux variation rate relative to the motor-shaft direction displacement is approximately represented proportional to a sine waveform that is different in phase by an electrical angle of 90° from the third phase coil current 103, and the product of the multiplication of the two values represents a motor-shaft direction force with respect to the third phase. Motor-shaft direction forces of individual phase coil currents of the first, second, and third phases are shown by waveforms 105, 106, and 107 in FIG. 13B. A synthetic motor-shaft direction force obtained by summing the motor-shaft direction forces 105, 106, and 107 of the three phases is shown by a waveform 108 in FIG. 13C. In the non-energized periods represented by Ta, Tb, Tc, Td, Te, and Tf, as shown in the synthetic motor-shaft direction force 108 in FIG. 13C, it can be known that vibration components of the shaft direction forces remain uncancelled. These result in residues of the vibration and noise.
In the example of FIGS. 13A to 13C, the shaft direction force remains in a period other than the above-described non-energized periods. This is because a current peak period (or, a current bottom period) representatively represented by 109 of FIG. 13A is long, and therefore the deviation from the sine wave having the trapezoidal waveform is large. This results in residues of the vibration and noise, similar to the above. As such, when the current peak period/current bottom period 109 is longitudinally steered to be an electrical angle of about 60°, the shaft direction force is reduced in a period other than the non-energized period.
In the motor driver circuit described in the conventional techniques, no driving transistor for directly driving the neutral point is connected in the “Y”-connected three phase motor coils. Accordingly, the total sum of the three phase coil currents becomes zero, and a coil-current freedom degree is 2. More specifically, when the coil current of one phase is set to zero to be in non-drive state, the freedom degree of the remaining two phases is only 1. Ordinarily, conventional driving methods are of the type restricted in the freedom degree, as described above. Consequently, according to motor driving with only the freedom degree of 2, for example, in the first-phase non-energized period Ta, the current values of the second-phase coil current 102 and the third-phase coil current 103 need to be identical to each other with the polarities opposite each other. This restriction makes it difficult to sufficiently reduce the vibration and noise of the motor including the non-energized periods.
According to the conventional configuration, the driving transistor for directly driving the neutral point of the “Y”-connected motor coils is not provided while the freedom degree of the three phase coil currents is 2, and therefore the residues of the vibration and the noise are sizable, that is, the residues are not sufficiently suppressed. Under constraints where, in the first-phase non-energized periods Ta and Te, both the current values, namely, the second-phase coil current 102 and the third-phase coil current 103, have the opposite polarities and the identical magnitudes, it can easily be inferred from the waveforms of the shaft direction force components attributed to the respective phase coil currents that the synthetic motor-shaft direction force 108 can never be sufficiently suppressed.